(1) Field of the Invention
The present invention relates to a contact-dehydrating sheet suitable for use in the preparation of dried or cured protein-containing food such as, for example, marine products (e.g. fishes) and meat for human consumption (e.g. beef, pork, lamb, chicken, game and the like). The present invention also relates to a method for preparing dried protein-containing food.
(2) Description of the Prior Art
It has been done worldwide from ancient times that protein-containing food such as fish and meat was dried by using sunlight to extend the preservative period of the protein-containing food. Recently, such drying has also been carried out by using fuel, such as heavy oil, instead of natural sunlight. That is to say, the air atmosphere temperature is raised directly or indirectly by the combustion heat energy of the fuels, whereby the relative humidity of the air atmosphere is decreased. In the air atmosphere having a decreased relative humidity and an elevated temperature, protein-containing food is hung up or placed on suitable racks generally composed of wire mesh or wire netting. in that:
(1) The drying step is largely affected by weather conditions which cannot be artificially controlled.
(2) The protein-containing food to be dried is unhygienically impaired or contaminated during the drying step by insects such as flies;
(3) The troublesome operations requiring much labor are included in hunging up the protein-containing food to be dried;
(4) In the case where the drying power is not sufficient to dry the protein-containing food at a desired drying rate (this frequently occurs due to cloudy weather and high humidity), the protein-containing food becomes half or nearly totally decomposed or stale conditions due to the fact that, since the drying rate (or water removal rate) of the protein-containing food is delayed, the active water content region, in which the activity of various bacteria is weakened, cannot be readily established in the body of the protein-containing food. Contrary to this, in the case where the drying rate is too high, a so-called "surface drying phenomenon" (which means only the surface portion of the substance to be dried is preferentially dehydrated to form the dried dense surface through which the transfer of water present in the body becomes difficult) occurs and, therefore, dried products having desired properties cannot be obtained. In order to obviate these problems, protein-containing food is generally dipped in an aqueous salt solution once or several times prior to the drying. However, this salt treatment produces very salty dried products, which are not desirable from the point of view of not only taste but also the generation of disease such as hyperpiesia.
Contrary to the case where the protein-containing food is dried by using sunlight, the above-mentioned problems can be solved by adopting a drying system in which artificially heated air or dried air is used in a so-called dryer for drying protein-containing food. However, this method still has disadvantages that:
(1) Fuel cost is very high, especially due to the recent increase in oil prices.
(2) Qualities of dried products become worse due to the fact that the hardening and oxidation of the protein are accelerated and autodigestion of the protein proceeds because of the increase in temperature.
(3) Amounts of available fresh food such as fish are not constant and, therefore, does not match the capacity of an enclosed drying system.
Furthermore, in either case, in the case where the protein-containing food is hung during drying, since the dried products shrink and the size of the protein-containing food becomes small, the commercial value of the products is decreased.
In addition to the above-mentioned drying methods, there is a drying method in which solid desiccant such as diatomaceous earth is used, although this method is not conventional. In this method protein-containing food is buried and dried in diatomaceous earth. However, since the dehydrating power or capacity of diatomaceous earth is relatively small in terms of the weight basis, an extremely large amount of diatomaceous earth, as compared with the weight of protein-containing food, must be used, which causes a lot of troublesome work and high cost. In addition, when the diatomaceous earth is used, only the surface portion of the protein-containing food is preferentially dried, a so-called surface drying phenomenon occurs. These problems also occur in the case where other solid desiccants similar to diatomaceous earth are used.
Furthermore, although a freeze-drying (or lypophilization) method can be theoretically applied to dry food, this is not practical for use in drying protein-containing food because the specific surface area of protein-containing food is small and, also, the decrease in the qualities of the products tends to occur because the flesh becomes spongy.
As a special method for preserving fresh protein-containing food, salt pickling and syrup pickling method may be applied. However, these methods have problems in that salt and syrup are transferred into the body of the protein-containing food to be dried, whereby the taste of the protein-containing food is remarkably changed and, also, that the dehydrating power of the salt and syrup solutions rapidly decreases as the dehydration from the protein-containing food proceeds, so that constant dehydrating power cannot be kept during the drying step.